1. Field of the Invention
The present invention relates to a novel apparatus which is used to measure the quantity of magnetite which has adhered to the walls of heat exchanger tubes which are commonly found in the heat exchanger vessels of nuclear steam generators and are also found in other heat exchangers, boilers, condensers or the like. A description of the internal mechanism of a heat exchanger vessel in a nuclear steam generator can be found in U.S. Pat. No. 4,320,528 issued on Mar. 16, 1982 to inventors Terry D. Scharton and G. Bruce Taylor. As discussed in that patent, during the heat exchange process, the high temperatures combined with the liquid environment caused the steel support plates which supported the heat exchanger tubes to oxidize to magnetite. Magnetite, whose chemical formula is Fe.sub.3 O.sub.4, was found at the juncture of the tube support plates and the heat exchanger tubes. The radioactive heat exchange process has also caused magnetite to form at other locations along the heat exchanger tubes. At the juncture of the heat exchanger tubes and support plates, the formation of magnetite caused denting and pinching of the tubes and eventually destroyed the usefulness of the tube which was dented or pinched. When magnetite forms at other locations along the heat exchanger tubes, the heat exchange process is impaired. It is therefore desirable to know how much magnetite has adhered to the heat exchanger tubes and the locations at which the magnetite has adhered.
In the example of nuclear steam generators, the heat exchanger vessel may contain as many as sixteen thousand thin walled, small bore tubes. By way of example, such tubes may have an outer diameter of approximately 5/8th of an inch and be sixty (60) feet long. They are also very tightly packed together inside the heat exchanger vessel. The only practical way to measure anything relating to these tubes is to place a very thin probe up one or more of the tubes and perform electronic measurements to determine the information that is desired. The present invention relates to the use of a novel type of probe to determine the locations along the length of one or more heat exchanger tubes where magnetite has adhered and to further determine the quantity of magnetite which has adhered at each of these locations.
2. Description of the Prior Art
Several different types of apparatus are known in the prior art for determining various parameters and other information about heat exchanger tubes, pipelines, oil well shaft, etc. by placing a probe into the physical structure and performing certain operations to determine the desired information.
There are several different prior art apparatus which are known for determining the physical dimensions of heat exchanger tubes and which are also used to determine abnormalities such as dents, out-of-roundness, holes, tears, and other problems with the tubes. These prior art apparatus use a probe which measures variations in eddy currents to determine the location and size of such abnormalities. An eddy current is an electric current induced by an alternating magnetic field. An eddy current is generated by a probe placed into and moved within the heat exchanger tubes, and the variation in the eddy current is combined with other data to determine the location and size of the abnormality. For example, the coil through which a sinusoidal current passes produces an electromagnetic field which induces eddy currents within the heat exchanger tube. These currents produce on return an alternating field which is set up in opposition to the initial field and then modifies the impedance of the coil. Any discontinuity which is present within the tube at the level of the probe modifies the path or intensity of the eddy currents. The defect is measured by various well known methods. Several prior art patents which address this type of art are:
1. U.S. Pat. No. 4,153,875 issued on May 8, 1979 to Pigeon et al. for "Eddy-current Testing Device for Metal Tubes Which Are Bent at Least Locally".
2. U.S. Pat. No. 4,325,026 issued on Apr. 13, 1982 to Cooper, Jr. et al. for "Plural Coil Eddy Current Mapping Probe".
3. U.S. Pat. No. 4,341,113 issued on July 27, 1982 to Gutzwiller, Jr. for "Inspection System for Heat Exchanger Tubes".
This method and apparatus is also discussed in informational literature prepared by Zectec, Inc., locted at 1320 N. W. Mall, Issaquah, Wash. 98027.
Other apparatus known in the prior art are used to test for magnetic material within long shaft like structures. U.S. Pat. No. 4,330,748 issued to Holden on May 18, 1982 for "Frequency Correction Circuitry for Pipeline Sensor Apparatus" discloses an apparatus used for the non-destructive testing of ferromagnetic materials in pipelines which are buried below the ground or submerged in water. An inspection vehicle is run along the outside of the pipeline which contains a probe containing an electromagnetic sensor to produce an electrical signal in response to variations in the magnetic field in the vicinity of the sensor and various electronic components to measure the magnetic signal read by the probe. U.S. Pat. No. 4,023,092 issued to Rogers on May 10, 1977 for "Apparatus for Sensing Metals in Wells" discloses an apparatus which located ferrous objects in well bores such as casings, collars and the like. U.S. Pat. No. 2,677,802 issued to Irwin on May 4, 1954 for "Method and Apparatus for Demagnetizing Material" relates to a method and apparatus for electrically testing the properties of materials which includes demagnetizing the material.
The apparatus in the prior art which comes closest to the present invention is disclosed in U.S. Pat. No. 4,088,946 issued to Charles et al. on May 9, 1978 for "Magnetic Bridge Transducer Formed with Permanent Magnets and a Hall Effect Sensor for Identifying the Presence and Location of Ferromagnetic Discontinuities Within or on a Tubular Specimen". The apparatus includes a probe which is placed inside heat exchanger tubes to measure magnetite. Although the object of this prior art apparatus is the same as the present invention, the specific technology employed is different. A key element of this prior art apparatus is a transducer which generates a magnetic field which can be placed in magnetic communication with and is movable over a portion of the surface area of the heat exchanger tube. A Hall element within the transducer is fixedly positioned with respect to the source of the magnetic field and arranged to be placed in magnetic communication with the specimen. The output of the Hall element which provides the transducer response is representative of the strength of the magnetic field having a component contribution from lines of force proximate and perpendicular to a given plane of the Hall element. Basically, in this prior art apparatus, the apparatus balances magnetic fields, within the probe itself, in such a way that a zero magnetic field is sensed at the Hall element when the probe is remote from external ferromagnetic materials. When a probe, initially balanced in this manner, is brought near a ferromagnetic material, the resulting electrical signal is a direct measure of the perturbation produced in the magnetic field and hence is also a measure of a quantity of ferromagnetic material present.
None of the apparatus disclosed in the prior art provides an effective and efficient means for measuring the quantity of magnetite which has adhered to heat exchanger walls. The apparatus in the Charles patent which employs a Hall element is subject to several inaccuracies. The major problem with the Hall element is that it measures an absolute magnetic field and does not allow for adjustments to particular environments such as different heat exchanger tubes which have different diameters, wall thicknesses and are made of different materials. In addition, the Hall element is subject to inaccuracies which result from variations of temperature and other properties in the environment.
In view of the difficult operating conditions under which workers must perform their tasks relating to measuring magnetite in the heat exchanger of the nuclear steam generator, it is necessary to provide an apparatus which is both simple and accurate and which permits measurements to be made at a location remote from the heat exchanger vessel. The embodiments in the prior art do not teach or make obvious such a device.